Stabilization of PVC

Whereas other metal salts, especially lead stearates and srdfates, or mixtures of Groups 2 and 12 carboxylates (Ba—Cd, Ba—Zn, Ca—Zn) ate also used to stabilize PVC, the tin mercaptides are some of the most efficient materials. This increased efficiency is largely owing to the mercaptans. The principal mechanism of stabilization of PVC, in which all types of stabilizers participate, is the adsorption of HCl, which is released by the PVC during degradation. This is important because the acid is a catalyst for the degradation, thus, without neutralization the process is autocatalytic. 

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

Tacticity or stereochemical arrangement of atoms in three-dimensional space in relation to each other along the polymer chain cannot really be termed a structural defect. But researchers have shown that tacticity has an important bearing on the reactivity and thermal stability of PVC. For this reason tacticity is being discussed under this section. 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

Horst mechanism of PVC stabilization. In fact, based on his observations Naqvi [134] has proposed an alternative model for the degradation and stabilization of PVC based on polar interactions within the polymer matrix, which will be discussed in the following section (Section VI). 

In conclusion, it may be said that a lot of literature has been published that favors the Frye and Horst mechanism of stabilization. Most of this is based on studies done on low-molecular weight model compound for al-lylicchlorines in PVC, i.e., 4-chloro-2-hexene. Although the large contribution of these studies toward understanding the mechanism of stabilization of PVC cannot be denied, the extrapolation of these results to the processes involved in the actual stabilization of the polymer should be done with extreme care. The polymer represents a complex mixture of macromolecules, which in the melt is not only physically a very different system compared to the low-molecular weight model compound, but invariably contains, apart from stabilizers, other additives, such as plasticizers, lubricants, processing aids, etc., that further complicate the situation. The criticism of the Frye and Horst mechanism is also based on solid experimental evidence, and hence, the controversy is still very much alive. 

Naqvi [134] has proposed an alternative model to the Frye and Horst mechanism for the degradation and stabilization of PVC. At room temperature, PVC is well below its glass transition temperature (about 81°C). The low thermal stability of the polymer may be due to the presence of undesirable concentrations of like-poles in the more or less frozen matrix with strong dipoles. Such concentrations, randomly distributed in the polymer matrix, may be considered to constitute weak or high energy spots in the polymer, the possible sites of initiation of thermal dehydrochlorination. 

Figure 2 Variation of thermal stability of PVC with acetate content.

This further interesting effect of very low levels of acetoxylation on the thermal stability of PVC may be explained on a basis similar to that applied to the copolymers, with specific reference to labile chlorines taken in isolation. But this can only be done with a certain amount of caution. 

Wirth and Andreas [141] studied the effect of octyl-tin chlorides on the thermal stability of PVC. They were found to significantly retard the dehydrochlorination in the following order ... 

Every alternating atom in the PDMS chain is the electronegative oxygen, which makes it a polar polymer. The authors observed that at low loadings of PDMS, PVC is destabilized. This is to be expected according to the alternative model for degradation and stabilization of PVC put forward by Naqvi. But for compositions with 50% or more PDMS, both polymers are stabilized. No possible explanation can be given for this reversal in thermal stability at this time. 

PBR increases the inherent stability of PVC by acting as a nonpolar dispersant and reducing the undesirable polar tensions within the PVC matrix. 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

E. Owen "Degradation and stabilization of PVC" Elsevier Appl. Sci. Publ. London 1984... 

Completely Reacted Material - The zinc borate contains no free zinc oxide. Thus, it does not have serious detrimental effects on the thermal stability of PVC or chlorinated paraffin as free zinc oxide does. 

Organic tin compounds have been used in various fields, such as antifouling paints, stabilizers of PVC, agricultural chemicals and so on, while lately anticancer and antitumor activities of organic tin compounds are also studied38,39. 

Polyvinyl Chloride. It is well-known how various attempts have been made to stabilise PVC against dehydrochlorination by salts, usually of divalent metal ions - as long chain alkylcarboxylates of Cd(II), Ba(U), Zn(II) [96]. Biswas and Moitra [102] recently established that the 3d metal ions incorporated in PVC-DMG-complex enhance the thermal stability of PVC in the order. 

Another very important property of plasticizers for PVC resins is the ability to act as a dispersing medium for polyvinyl chloride plastisols. The performance of the particular plasticizer in a plastisol application depends to a great extent on a property known as its viscosity stability. The stability of the plastisol to viscosity build up is the manifestation of the solvating effects of the plasticizer, and thus, those materials with higher solvating efficiency tend to display an increase in plastisol viscosity on standing. Figure 3 is the plot of the viscosity stability of PVC o-phthalate ester plastisol compositions and, in essence, compares the alcohol-derived plasticizers with that of the olefin-derived plasticizer. The data show that... 

The improved thermal stability of the chlorinated paraffins also reflects the purity of the n-alkanes. The heat stability of PVC plasticized with chlorinated n-paraffin at 177°C. is better than that of similar compounds using commercial chlorinated wax. This is shown in Table I by the color that developed during thermal treatment for samples in which 40% of the dioctyl phthalate was replaced by chlorinated paraffins. (In this example and those that follow, the total plasticizer concentration is constant at 33 / %.) The color development in this test is significant as a measure of the comparative processing conditions that can be employed. 

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